1. Field of the Invention
The present invention relates generally to an electrically power-assisted four-wheel steering system for vehicles, and more particularly to an electrically power-assisted four-wheel steering system for vehicles including a front-wheel steering device of a rack and pinion type, a rear-wheel steering device mechanically connected to the rack of the front-wheel steering device, and an electric motor for operatively providing auxiliary torque to the steering devices.
2. Description of Relevant Art
The present applicant has disclosed in UK Patent Application Publication No. GBA 2148812, published June 5, 1985, a four-wheel steering system for vehicles including a front-wheel steering device of a rack and pinion type and a rear-wheel steering device mechanically connected to the rack of the front-wheel steering device. The rack has an end portion thereof formed with a first toothed part meshing with a pinion which is connected to a steering wheel. In a central portion of the rack there is formed a second toothed part meshing with another pinion that is provided at the front end of a long link which longitudinally extends at a transversely central position of the vehicle. To avoid unnecessary increase in vehicle height, the long link is accommodated in a channel formed in a rise into the passenger room, and transversely centrally positioned with respect to the vehicle. The rear end of the link is connected through a steering angle ratio changing mechanism to tie-rods and knuckles of rear wheels. The ratio changing mechanism, as well as the tie-rods and knuckles, constitutes the rear-wheel steering device. The gear ratio of this mechanism is controlled such that the rear wheels are turned in the same direction as front wheels when the vehicle speed is high, and in the opposite direction to the front wheels or in neither direction when the vehicle speed is low.
In U.S. Pat. No. 4,415,054 there is disclosed an electric power steering system for vehicles in which auxiliary torque from an electric motor is transmitted through a ball and screw mechanism to the rack of a rack and pinion type front-wheel steering device. The motor is coaxially fitted on a central part of the rack. The ball and screw mechanism consists of a screw portion spirally formed along the entire outer circumference of the central part of the rack, a ball bearing nut fitted on the screw portion and formed with a spiral groove along the inner circumference thereof, and many balls interposed between the screw portion and nut. The outer circumference of the nut is directly fixed to or indirectly connected through a clutch to an armature of the motor. Power and rotation of the motor is transmitted to the rack through the ball and screw mechanism, where the speed of rotation is reduced. One end part of the rack is formed with teeth meshing with a pinion which is connected to a steering wheel. The ball and screw mechanism transmits auxiliary torque from the motor with a small friction loss, thus permitting a smooth transmission with high efficiency.
It is desirable to combine advantages of the four-wheel steering system with those of the power steering system including the ball and screw mechanism, to thereby provide an electrically power-assisted four-wheel steering system. The rack in the front-wheel steering device of the four-wheel steering system however has in the central portion thereof the toothed part already formed to mesh with the pinion of the long link, which constitutes the difficulty of forming the screw portion of the ball and screw mechanism along the entire outer circumference of the central portion of rack.
The present invention has been achieved to overcome such a problem and effectively materialize such a desideratum as described.